This invention relates to a novel circuit constructing technique for direct conversion reception in a signal radio receiver. More particularly, the present invention relates to a novel circuit constructing technique for direct conversion reception in a FSK (Frequency Shift Keying) signal radio receiver.
In a conventional FSK signal radio receiver, generally, a high-frequency signal which was received by the receiver through its antenna is converted in frequency to an IF (Intermediate Frequency) signal using a superheterodyne system, and the IF signal thus frequency-converted is frequency detected by a frequency discriminator. This method can provide the major portion of amplification degree and selectivity requisite for the receiver, only in the IF amplifier stage. However, with this method, there is the problem that in order to avoid image frequency disturbance which would be produced, an increase of the selectivity of a high-frequency amplifier stage is needed to a some extent, which may be provided at a receiver stage preceding the heterodyne frequency-converting stage, and therefore its structure becomes inevitably more complex as the the reception band of the receiver, within which FSK signal frequencies can be received, is widened. Moreover, the tuned frequency tracking at each of the high-frequency amplifiers in the high-frequency amplifier stage and a local oscillator becomes much more difficult.
By contrast, in recent years, a direct conversion receiving method has been put to practical use, which is suitable especially for the reception of a FSK signal, since a radio receiver for the FSK signal can be constructed quite simply. This is based on the reason that in the direct conversion system, the received signal by the receiver is directly frequency-converted, or dropped in frequency, to the frequency of its base band signal (this means here a signal of which center frequency is merely beaten down to zero, not the base band signal which is the modulation signal). As a result, two axis component signals (in-phase and orthogonal phase) are produced, and these signals are amplified respectively. The phase difference between these two signals is +.pi./2 or -.pi./2, depending upon whether they are high or low in frequency with respect to the center frequency of the input signal, which has its phase value which is a two-level value.
In the case of the FSK signal, since information carried by the FSK signal is in itself the two-level value, in its demodulation, the two-level value which the base band signal phase has can be used directly. Furthermore, in the case of the FSK signal, since, as to a phase error between amplifiers for both the base band signal components, a limiter can be used to limit the input signal, additionally, its output, which is in the form of a rectangular waveform, can be reproduced merely by means of direct logical judgment, and therefore it is possible to construct a receiver with extreme ease for such.
Following the development of the direct conversion FSK signal receiving system, dedicated LSI (Large Scale Integration) circuits for the FSK signal radio receivers, according to the direct conversion FSK signal system, have been put to practical use. However, the input signal is directly converted, amplified and then demodulated using a logical circuit, without the use of an AFC (Automatic Frequency Control) circuit for correcting any frequency error in the input signal or local carriers. As for an AFC for the FSK signal, it is required that the AFC operate with respect to the center frequency of the FSK signal. However, usual AFC circuits, which are used widely at present, merely operate so as to make any frequency error equal to zero at every moment. Therefore, if the AFC signal is modulated so that it continues for a long time at one of its two frequencies, the AFC is inevitably captured to its frequency. Accordingly, in case where the conventional AFC circuits would be utilized with the direct conversion FSK receiver LSI circuits, as they are, the error code rate would be increased depending upon the contents of modulated codes.
Until now, various LSI circuits have been developed for simplifying the construction of a direct conversion FSK signal radio receiver. However, such LSI circuits can be used only in case where there is not reception frequency error. If there is a frequency error, code distortion would be produced and the noise margin would be decreased. Even though a typical AFC was applied to such construction in order to improve the disadvantage, many character errors were produced. Since the FSK signal itself varies its frequency due to modulation, the AFC can follow the moving average value, and in the case of a long code, the AFC can be captured to that frequency, and as a result many character errors were produced.
In view of the above, the present invention is intended to solve the above problems with very simple means, and to provide a direct conversion FSK signal receiver having a stable reception operation all the time.